The application of insulating coatings to conductive substrates to produce coated sheets useful in electrographic printing processes is well known. In the known process, a paper which has been impregnated to render it electrically conductive is coated on one surface with an insulating resin which contains a proportion of inexpensive pigment, such as calcium carbonate, to provide an attractive surface coating which will hold an electrostatic charge. The coated paper is then passed over a charging electrode which applies an electrostatic charge to the coated surface in a pattern, and the coating is expected to receive as high a charge as possible and to hold this charge so that toner will be picked up only in the charged pattern.
The existing electrographic coatings are inadequate because: 1-they do not accept and hold as high a level of charge as is desired; 2- they tend to pick up a background charge; and 3- they cannot usefully contain as high a proportion of pigment as is desired.
Some of these problems were overcome in our prior application referred to hereinbefore, but solution coatings were there employed. Solution coatings have the evident disadvantage of employing organic solvents which are costly and which introduce a fire hazard. Moreover, solution coatings tend to penetrate the paper, and this normally requires that the paper be precoated to minimize penetration. Aqueous emulsion coatings avoid the expense and hazard of organic solvents, and they do not penetrate the paper as much, so the prior pretreatment can be eliminated. Unfortunately, most aqueous emulsion coatings do not provide very effective electrographic coatings.